It appears that the PDFLib monochrome output will be in the form of a CMYK file. However all colours are converted to tints of black. If your customer requires a "true grayscale" (non-CMYK) PDF file then I don't believe that can come directly from Styler (APP) itself. There are plenty of PDF tools capable of manipulating the file after it comes out of Styler (APP). Even Acrobat can do colour-to-grayscale. Another idea is to have the customer implement a preflight step where they check the PDF for non-black ink usage.
BTW if you test with FOSI engine I believe this will use RGB instead.